LibWeb: Make LayoutState use HashMap instead of potentially huge Vector

Before this change, LayoutState essentially had a Vector<UsedValues*>
resized to the exact number of layout nodes in the current document.

When a nested layout is performed (to calculate the intrinsic size of
something), we make a new LayoutState with its own Vector. If an entry
is missing in a nested LayoutState, we check the parent chain all the
way up to the root.

Because each nested LayoutState had to allocate a new Vector with space
for all layout nodes, this could get really nasty on very large pages
(such as the ECMA262 specification).

This patch replaces the Vector with a HashMap. There's now a small cost
to lookups, but what we get in return is the ability to handle huge
layout trees without spending eternity in page faults.

Userland/Libraries/LibWeb/Layout/Node.cpp

@@ -32,8 +32,6 @@ Node::Node(DOM::Document& document, DOM::Node* node)
     , m_browsing_context(*document.browsing_context())
     , m_anonymous(node == nullptr)
 {
-    m_serial_id = document.next_layout_node_serial_id({});
-
     if (node)
         node->set_layout_node({}, *this);
 }